1.31 -- Rewrite of map2stan to use distribution templating

map2stan substantially improved
- can resampling from existing compiled model
- allows multiple likelihoods and linear models
- so therefore allows zero-inflated models
- added dmvnorm2 that takes vector of sds and Rho, so priors can be
specified independently
- so added lkj_corr distribution for Rho priors
- added dinvwishart cov_matrix priors
- added dlkjcorr and rinvwishart R functions
- fixed plotchains and optional parameters
- updated man page and examples
- places old map2stan algorithm in map2stan_old (marked for later
deletion)